Richard L. White

Nanoindentation and Nanoscratching of Hard Carbon Coatings for Magnetic Disks

Nanoindentation and nanoscratching experiments have been performed to assess the mechanical properties of several carbon thin films with potential application as wear resistant coatings for magnetic disks. These include three hydrogenated-carbon films prepared by sputter deposition in a H{sub 2}/Ar gas mixture (hydrogen contents of 20, 34, and 40 atomic %) and a pure carbon film prepared by cathodic-arc plasma techniques. Each film was deposited on a silicon substrate to thickness of about 300 run. The hardness and elastic modulus were measured using nanoindentation methods, and ultra-low load scratch tests were used to assess the scratch resistance of the films and measure friction coefficients. Results show that the hardness, elastic modulus, and scratch resistance of the 20 and 34% hydrogenated films are significantly greater than the 40% film, thereby showing that there is a limit to the amount of hydrogen producing beneficial effects. The cathodic-arc film, with a hardness of greater than 59 GPa, is considerably harder than any of the hydrogenated films and has a superior scratch resistance.

Materials Issues in Magnetic-Disk Performance

The continued exponential growth in areal density for longitudinal magnetic-recording devices places ever more stringent demands on disk performance. The design of materials and processes must provide the required advances in technology. The magnetic properties are controlled through the choice of underlayers, magnetic alloys, and the deposition processes that control crystallographic orientation and magnetic isolation between grains. The requirement of lower head-disk spacing places increasing stress on the tribological performance of the disks, controlled by a very thin overcoat and lubricant layer. This article reviews the various materials issues relevant to magnetic-disk technology. The major obstacle for achieving high areal density in thin-film media is transition noise. This noise arises from the zig-zag transition boundaries that occur due to cooperative switching of the magnetic grains. Both exchange coupling between grains and magnetostatic interactions cause magnetic-cluster sizes larger than the grain size. The goal is to magnetically isolate the grains and keep the grain size small. As the dimensions of the bit cell shrink, smaller grain size is required to obtain enough grains per bit cell to maintain the required signal-to-noise ratio (SNR). In the 10–40-Gbits/in. 2 areal-density range, the issue of thermal stability of small ( In addition to good SNR, a narrow transition width is needed in order to pack the transitions closer together. The objective is to minimize interactions between transitions to reduce nonlinear amplitude loss and superlinear noise.

Coverage and properties of a-SiNx hard disk overcoat

Amorphous silicon nitride (a-SiNx) overcoats are deposited on magnetic disks by rf-reactive sputtering to study their coverage and properties. According to the x-ray photoelectron spectroscopy analysis, a-SiNx has a low coverage-limit of ∼10 A compared with that of the reference a-CNx (∼20 A). The lower coverage-limit of a-SiNx may be attributed to its high density of 3.2 g/cm3, which corresponds to 93% bulk density. By contrast, the density of diamond-like carbon is only 54% that of diamond. This is in agreement with the results of coverage simulation, which reveal that the film coverage thickness decreases by ∼3 A per 10% increase in the relative density. Compared with 45 A a-CNx coated disks, 15 A a-SiNx coated disks have fewer pinhole defects and are more durable in the accelerated flyability test. The superior performance of a-SiNx disk overcoat may be attributed to its dense structure and high hardness (25 GPa).

Microstructure and properties of ultrathin amorphous silicon nitride protective coating

The effect of N content on the structure and properties of rf reactively sputtered amorphous silicon nitride (a-SiNx) has been studied by Rutherford backscattering spectrometry, x-ray reflectivity, ellipsometry, and nano-indentation. The N content in the film increased with the N2 concentration in the sputtering gas until the Si3N4 stoichiometry was reached. The hardness of a-SiNx increased with density, which in turn increased with the N content. The maximum hardness of 25 GPa and density of 3.2 g/cm3 were attained at the stoichiometric Si3N4 composition. With the application of a protective overcoat for magnetic disks in mind, thin a-SiNx films were deposited on CoPtCr media to examine their coverage, pinhole density, and wear resistance. According to x-ray photoelectron spectroscopy, the minimum thickness of a-SiNx required to protect the CoPtCr alloy from oxidation was 10 A, which was 10 A thinner than that of the reference amorphous nitrogenated carbon (a-CNx). A statistic model showed this lower thi...

The effect of hydrogen in carbon overcoats on the tribology of the head-disk interface

The effect of hydrogen in the carbon overcoat on the CSS durability of thin film media lubricated with perfluoropolyether (PFPE) is reported. A strain-gauge force transducer and optical surface analyzer (OSA) were employed to investigate the failure mechanism. The OSA provided in-situ monitoring of lubricant migration and modification as well as carbon wear. Durability improved with increasing hydrogen content over the range from 12 to 36 at% of H. The improvement is attributed to changes in lubricant bonding and mobility as a function of hydrogen content.

Effect of N doping on structure and properties of DLC films produced by plasma beam deposition

A novel plasma beam source for the deposition of DLC films is described. Wide ranges of ion energy (130-250 eV) and C/sub 2/H/sub 2//N/sub 2/ flow conditions have been used to investigate the effect of N doping an the structure and properties of DLC films. The resulting films are characterized by their chemical composition, Raman spectra, electron spin density, mass density, and hardness, which critically depend on the N content. The addition of N causes the sp/sup 2/ carbon content in the DLC films to increase and results in lower density and hardness. The film density also decreases with increasing ion energy at high N concentrations. Carbon films with maximum density and hardness of 2.1 g/cm/sup 3/ and 25 GPa, respectively, can be produced using the plasma beam source.

The interaction of short-chain model lubricants with the surfaces of hydrogenated amorphous carbon films

The adsorption of water and small model perfluorinated lubricants on hydrogenated amorphous carbon (a∶C-H) films of varying hydrogen content was investigated using thermal desorption spectroscopy (TDS). Hydrogen content of the carbon films was measured by Rutherford back scattering (RBS) and elastic recoil spectroscopy (ERS) and correlated to changes in surface free energies measured by contact angle analysis. Hydrogenated carbon films exhibiting the highest surface free energy provided a greater attractive interaction for the model lubricants. All model lubricant species studied - water (D2O), perfluorodiethyl ether (CF3CF2OCF2CF3), perfluoropentane (CF3(CF2)3CF3), perfluorooctane (CF3(CF2)6CF3), 2,2,2-trifluoroethanol (CF3CH2OH), and 1,1,7-H-perfluoroheptanol (CF2H(CF2)5CH2OH)—reversibly adsorbed to the carbon surface with little chemical reaction. Increases in desorption energies with increasing chain length were observed among the adsorbates and are ascribed to increasing van der Waals interactions. Incorporation of alcoholic end groups provided an avenue of hydrogen bonding to the surface and produced an ~20 kJ/mol increase in desorption energy relative to a perfluorinated alkane of the same chain length. Ether linkages within the model lubricant provide little increase in desorption energy as fluorine substituents effectively screen the oxygen. Together these findings implicate a predominantly physisorbed state for perfluorinated lubricants on hydrogenated carbon surfaces.

Nanoindentation and nanoscratching of hard coating materials for magnetic disks

Nanoindentation and nanoscratching experiments have been performed to assess the mechanical and tribological behavior of three thin film materials with potential application as wear resistant coatings for magnetic disk storage: (1) hydrogenated-carbon (CHx); (2) nitrogenated-carbon (CNx); and (3) boron suboxide (BOx). The hardness and elastic modulus were measured using nanoindentation. Ultra-low load nanoscratching tests were performed to assess the relative scratch resistance of the films and measure their friction coefficients. The mechanical and tribological performance of the three materials are discussed and compared.

Mechanical Properties of Carbon Films for Thin Film Disks

Carbon overcoat films are used extensively in thin film disk applications to provide wear resistance. A nano-indentation technique and wafer curvature measurements have been used to study the mechanical properties of carbon films sputtered under various processing conditions. Specifically, the effects of substrate/target spacing, power, pressure, and substrate bias have been studied for films sputtered in an argon plasma. The relationship of these properties to contact start-stop performance of hydrocarbon lubricated disks is further described. The frictional performance during the test can be related to film hardness, while the durability can be affected by the residual film stress.

Nanoindentation and the tribology of head-disk interface components

Nanoindentation data are presented for three different but interrelated components of the thin film disk magnetic recording system-the air bearing slider/recording head, the disk substrate, and the sputter deposited thin film disk recording medium. Hardness traces across the slider trailing edge demonstrate the hardness of the NiFe, sputtered Al/sub 2/O/sub 3/, and the ceramic Al/sub 2/O/sub 3//TiC are 8, 10 and 24-40 GPa, respectively. Lapping under non-optimum, aggressive conditions can lead to significant recession in these components which is directly related to their hardness. The hardness and modulus have been measured for a number of alternate substrate materials ranging from AlMg/NiP to glass and glass ceramic. The ability of these substrates to resist damage from slider shock forces is presented and generally increases with substrate hardness, although other criteria (fracture toughness and plasticity initiation) rue required to rationalize all the data. Finally, hardness and modulus of carbon overcoat films are presented which have been sputtered under various conditions. The process variations lead to variations in hardness, the hardness/modulus (H/E) parameter, and tribological performance in slider/disk testing. The applicability of these mechanical property parameters to the wear degradation is discussed.